Covariance and correlation coefficient

In summary, the maximum value of 2*cov(x,y) can be equal to var(x) + var(y) and the correlation coefficient, cov(x,y)/(sigma(x)*sigma(y), can only be between -1 and 1 due to the Schwarz inequality, which states that |E(XY)|^2\le E(X^2)E(Y^2), where X and Y are the random variables centered at their means. This has been proven and can be easily found through a Google search or by using the Cauchy-Schwarz inequality.
  • #1
Josh S Thompson
111
4
How do you prove that the maximum value of 2*cov(x,y) can be is equal to var(x) + var(y).

Moreover, how do you prove that the correlation coefficient, cov(x,y)/(sigma(x)*sigma(y), can only be between -1 and 1.
 
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  • #2
Forget the first sentence the question is only the second sentence
 
  • #3
Try computing a bunch for random and real data sets and you will see that the rule is never violated.

Of course, I'm an experimentalist.
 
  • #4
is there a proof?
 
  • #5
Josh S Thompson said:
is there a proof?

I bet there is for a statement for which no counter example exists. I think I recall even seeing one when I taught statistics.

But you can probably Google it up as easily as I can.
 
  • #6
Google sucks, I want some pictures bro. Because I did some examples and I don't understand, I think it doesn't violate those rules because of like dot products or something but I don't see the correlation coefficient. Can someone please enlighten me with some insight.
 
  • #7

1. What is the difference between covariance and correlation coefficient?

Covariance is a measure of the direction and strength of the relationship between two variables. It indicates how much two variables change together. The correlation coefficient, on the other hand, is a standardized measure of the same relationship, which takes into account the scales of the variables and ranges from -1 to +1.

2. How do you interpret a positive covariance or correlation coefficient?

A positive covariance or correlation coefficient indicates a positive relationship between the two variables. This means that as one variable increases, the other variable also tends to increase. The strength of the relationship depends on the magnitude of the covariance or correlation coefficient.

3. What does a negative covariance or correlation coefficient mean?

A negative covariance or correlation coefficient indicates a negative relationship between the two variables. This means that as one variable increases, the other variable tends to decrease. Again, the strength of the relationship is determined by the magnitude of the covariance or correlation coefficient.

4. How do you calculate covariance and correlation coefficient?

To calculate covariance, you need to first calculate the mean of both variables. Then, for each data point, subtract the mean of each variable from its value and multiply them together. Finally, add up all of these values and divide by the total number of data points. To calculate correlation coefficient, you need to first calculate the covariance. Then, divide the covariance by the product of the standard deviations of the two variables.

5. Can covariance or correlation coefficient be used to determine causation?

No, neither covariance nor correlation coefficient can be used to determine causation. They only measure the relationship between two variables, but do not indicate which variable is causing the other. Causation can only be determined through further experimentation and analysis.

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